Michell Instruments
has produced an absolute humidity and relative humidity slide
rule.

It was made with
the customer in mind and provides the user with a fast, clear
and simple slide rule formula. The useful tool also includes
practical definitions and equations such as the equation for
ppmv and the way of calculating a given saturation vapour pressure.

The front, Michell
side of the slide rule calculates absolute humidity allowing
the user to align measured dewpoint value with the operating
pressure and then read the moisture content (ppmv) clearly and
concisely. The user can then reverse the process to calculate
dewpoint at pressure from ppmv. The reverse, or Rense
side of the calculator allows the user to set temperature (oC
or oF) at a given relative humidity and then calculate the dewpoint.
This process can then be reversed to calculate % rh at temperature
from dewpoint.